ALUMINIUM AND ITS ALLOYS
Imitation plate for table ware, trays, etc., consists of argentan with a
low proportion of nickel (7-10%) heavily silvered galvanically (it contains
2-3% Ag). Besides the trade-mark, it often exhibits—particularly with
forks, spoons, etc.—a number indicating the quantity of silver deposited
per dozen pieces. The alloy is analysed like argentan. Usually, however,
it suffices to determine only the layer of silver and this may be effected as
Determination of the Silver.—i. After being well cleaned and freed
from grease, the object is suspended by a platinum wire in a 2-3% potassium
cyanide solution in a tall, narrow cylinder and is connected with the positive
pole of a current source. A thin, clean copper sheet in communication
with the negative pole, is also suspended in the liquid but not in contact
with the object. The current (o-1-0-2 ampere) dissolves the silver from
the article and deposits it on the copper. When the de-silvering is com-
plete, both the object and the silvered copper are removed and washed,
the latter being dissolved in nitric acid, the solution diluted and the silver
precipitated by a slight excess of hydrochloric acid; the silver chloride is
collected in a Gooch crucible, washed, dried and weighed.
The hydrocyanic solution is acidified with dilute hydrochloric acid
(under a hood], the liquid evaporated until cyanogen compounds are com-
pletely eliminated, and the precipitated silver chloride weighed. From
the sum of the two quantities of silver chloride the amount of silver on the
object is calculated.
2. The article, or part of it, is freed from grease, weighed, and gently
heated with a mixture of 9 vols. of cone, sulphuric acid and I vol. of cone,
nitric acid. By this means all the surface silver is rapidly dissolved, whilst
the metal beneath is not at all or but little attacked. When the de-silvering
is complete, the object is withdrawn, washed rapidly and thoroughly with
water,1 dried and weighed. The loss in weight gives the silver plating.
For a more rigorous determination the silver dissolved may be esti-
mated by diluting with water the nitric-sulphuric solution, together with
the washing water, and determining the silver either volumetrically by
Volhard's method or gravimetrically as chloride.
ALUMINIUM AND ITS ALLOYS
Owing to its lightness and stability, aluminium is now used for making
many diverse objects in common use and for naval and flying construction.
Further, aluminium forms a constituent of numerous alloys, many of which
are mechanically superior to pure aluminium. Among these are : Light
aluminium-bronze (Al with 3-8% Cu) ; Magnalium (Al with 3-15% Mg) ;
Barbouze's alloy (Al with 10% Sn) ; Z-iskon (Al with varying proportions
of zinc) ; aluminiiim-nickel (Al with 1-3% Ni) ) aluminium-manganese
1 Thorough washing is effected by taking the object quickly from the acid mixture
and immersing it in a fairly large vessel full of water.ric Analysis of Ordinary Brasses.